laminated ribs

[Dave McKibben]

Hello list:

I wont go into a dissertation, but stress and strain are both terms related
to mechanical parts and failure.
There is such a device called a ' strain gage ' that when attached to a
mechanical part will measure strain.  It is a ' unit- less ' value of
micro-inch / inch, and is usually referred to as micro-strain.  It
basically measures how many microinches does a material move   /   per inch
of total material  when subjected to a given force.
Generally speaking.....  :
All materials have an elastic region.  A state where when a force is
applied, the material will stretch, but when the force is taken away, the
material will rebound back to its original shape.  There is another term
related to this region called ' modulus of elasticity ' .
All materials have a plastic region. A state where when a force is applied,
the material will stretch, but when the force is taken away, the material
will NOT rebound completely back to its original shape. It has been
permanently deformed and its structure has been altered.
And of course all materials have a region where they will break when a
force is applied.

These values of  a materials ability to ' stretch ' are measured with
strain gages and other devices and that is how the #'s in engineering books
are formulated.  These #'s are then used for design puroposes to make sure
parts TYPICALLY are never used OUTSIDE its particular ELASTIC region.
Sooooooo.........there is tensile force and strain,   Torsional force and
strain,  shear force and strain, and bending force and strain and as you
would guess some parts have multiple stresses and strains in them when put
to use.
I use to work in a failure analysis lab and have put on and wired thousands
of strain gages.  Mostly on metals.
I'm not sure this has any real value here, but there it is anyway.
Have a great week !

Dave

> [Original Message]
> From: David Love <davidlovepianos at comcast.net>
> To: Pianotech List <pianotech at ptg.org>
> Date: 4/3/2006 6:00:05 PM
> Subject: RE: laminated ribs
>
> The problem, I believe, is that "strain" is not an engineering term that
I'm
> familiar with and its specific meaning in the description below is vague.
>
> David Love
> davidlovepianos at comcast.net 
>
> -----Original Message-----
> From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On
Behalf
> Of Ric Brekne
> Sent: Monday, April 03, 2006 2:04 PM
> To: pianotech at ptg.org
> Subject: laminated ribs
>
> Hi Dale
>
> This argument about ribs not supporting crown has bothered me from the 
> first moment I heard it. And after reading Nossamans well written 
> article in the latest Journal  I think I know why.  Ok, nobody questions 
> that in a CC board ribs do not provide beam support for the load. But 
> thats not quite the same thing as saying they dont support load in a 
> different fashion.  The same thing goes for the crown arguement. And 
> thats where Rons article comes in.
>
> About 2 years ago I posted a couple threads with some drawings trying to 
> explain why I thought the ribs in a CC board had similiarities to a 
> cable in the sense that they attempt to constrain the board from 
> expanding... so the panel has to bend instead. That very resistance to 
> the panels expansion is every bit as much a load support but in an 
> entirely different way. What the kicker back then was, was that I tried 
> to argue that the ribs strain (note the word usage) against the 
> expansion forces from the panel.  I was told then that no.. the ribs 
> dont strain.... they simply bend against their better nature.  Enter 
> Rons article disclaiming the buttress arch.  In that article he shows by 
> experiment that the top half of the ribs not only bend, but they expand 
> lengthwise. That expansion is critical to his whole arguementation 
> (which by the way made perfect sense to me).
>
> But that same rib expansion shows conclusively that the ribs do strain, 
> and significantly so against the expanding panel.  If you stop to think 
> about it this only makes sense.  If the ribs can not strain lengthwise 
> at all, then neither could the panel crown, yet if they strained equally 
> through their height then they would not constrain the panel at all.  It 
> is because they DO strain ... more on top and increasingly less towards 
> the bottom combined with the panels compression that crown and crown 
> strength occur. And it doesnt really seem to me to be so much a stretch 
> of the mind to imagine mathematical explainations for all this that 
> would fit very nicely into design thinking.  The height and width of 
> rims dont add up to combine in a kind of beam strength / mass 
> relationship... but rather a kind of strain strength / mass one.  
>
> One thing is clear about load support in CC boards. The more you push on 
> it, the more it resists... until its overloaded of course.  But until 
> that point there is definately load support and the ribs are definatly 
> part of that... just not in the sense of beams.
>
> Cheers
> RicB
>
>
>
>
>
>
> ------------------
>
> Dale,
> Those ribs were originally built into a CC board. How can  a
> compression crowned board get mechanical support from the rib
> scale,  however "good" the scale looks? The ribs in CC boards
> resist the crown  that panel compression is trying to form and
> maintain, and just put more  compression load on the panel.  -----Ron
>  
>   No, I get all that Ron, but if the ribs are built significantly  taller
&
> of stiffer material by design  then more panel  compression can be taken 
> out of
> the equation.  Another thought is, &  I've witnessed this quite a few
times
> is, that Stwy A's (1 & 2"s) in  general can produce a pretty wonderful 
> sound
> even with a  flat or  flattish board providing there is some small but
> consistent bearing load  still intact so in this case it would seem that 
> there are
> enough impedance  factors about the rib scale to make the system work 
> rather well.
> Ok maybe a  freak of nature but it happens fairly frequently.  
>    About 5 years back I had such an long A I was going to  resell it. It 
> had
> Steinway hammers which were quite soft &  made  it sound really good.  
> It was
> hard to imagine that a new board would make  it sound much better. It
truly
> sounded glorious but it was a spec job  & I don't sell old boards very 
> often. I
> didn't do any thing to the  action until later for a really good A b
> comparison.
>    So  I built a board with the same number of ribs  making them crowned
at
> about 60 ft. Made em taller but not much. Used sugar  pine in the bottom &
> yellow pine in the top.  The sound was  cleaner and the sustain was 
> about the same
> which was awesome.  It just  had it!!
>   I attribute much of this to the original basic rib scale  design. 
> Something
> was working or several things were.  Do you see what  I'm saying?
>   I'll crunch some numbers & see what I got.  I greatly  Appreciate the
> design sharing & information swap.
>   Thanks
>    Dale
>
>
> It's  an entirely different system. Do a bearing load analysis
> on the ribs as  load carrying beams and see what the numbers
> say. They'll say that the rib  scale isn't adequate to support
> bearing without substantial panel  compression support.
>
>
>
>